Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

R Parnell-Turner,R.C Searle,C.J Macleod,R.A Sohn,N.M Simão,C Peirce,T.J Reston

doi:10.1130/g48420.1

Abstract

AbstractAt slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.

Highlights

At spreading ridges with a low or variable magma supply, faulting is commonly heterogeneous, giving rise to a variety of deformation styles, including long-lived detachment faults (Cannat et al, 1995; Blackman et al, 1998; Escartín et al, 2003; Ildefonse et al, 2007; MacLeod et al, 2009)
High levels of persistent seismicity along the basal portion of the detachment surface have been observed at the Trans-Atlantic Geotraverse (TAG) detachment on the Mid-Atlantic Ridge at 26°N, suggesting that this type of activity may be common to active oceanic detachment faults
Our results provide new insight into the subsurface fault structures associated with the Subsurface Fault Structure and Linkages We observed a seismic gap between the two oceanic detachments in both the 2014 and 2016 deployments

Summary

Introduction

At spreading ridges with a low or variable magma supply, faulting is commonly heterogeneous, giving rise to a variety of deformation styles, including long-lived detachment faults (Cannat et al, 1995; Blackman et al, 1998; Escartín et al, 2003; Ildefonse et al, 2007; MacLeod et al, 2009). The first micro-earthquake survey was an approximately 6 month experiment from April to October 2014, with 25 short-period ocean-bottom seismographs (OBSs) deployed along ∼10 km of the ridge axis, which yielded new insight into the internal deformation of the fault footwall (Parnell-Turner et al, 2017).

Results

Conclusion